This invention relates generally to digital encoding apparatus and more particularly to linear digital shaft encoders for use in measuring the relative movement of two parts of a mechanical assembly.
Digital shaft encoders have commonly been used in the prior art, but such units are almost exclusively rotary encoding units which measure the degree of revolution of a shaft by using a transparent disc mounted to rotate with the shaft, the disc passing between the elements of a photooptical system for sensing light and dark areas on the transparent disc. Such encoders are most typically included as an element in the overall machine design so that the encoding disc can be placed upon a shaft which is properly aligned within the machinery. In such installations, alignment is not a significant problem. Moreover, it is well recognized in the encoder art that, assuming that the rotating shaft is straight, alignment problems do not occur, since the shaft will always rotate about its own axis and the photooptical system need only be rigidly mounted relative this fixed axis to assure alignment.
Even with rotary shaft encoders which are not designed as a part of the overall equipment but are added onto a rotating shaft, it is a simple matter to set a rotating shaft within the encoder unit within bushings and to use a coupling to connect this shaft to the main rotating shaft, the bushings within the self-contained rotary digital encoder assuring alignment of an enclosed rotating transparent disc and a photooptical system.
Much more severe difficulties have been incurred in the prior art, however, in the field of linear shaft encoders whereby the linear relative distance between a pair of machine elements is to be monitored. Because of severe alignment problems encountered in such encoders, the prior art has generally taken two diverse approaches, neither of which is entirely satisfactory. The first approach commonly utilized in the prior art is to measure such linear dimensions using a rotary encoder turned by a lead screw. Thus, the linear dimension, through mechanical linkages, is transformed into a rotary motion which is then measured. While, as discussed above, such rotary encoders do not suffer the alignment dificulties of linear encoders, the inaccuracies which result in this type of linear encoding system are substantial. Thus, for example, the lead screw mechanism may have substantial backlash which increases the error of the monitoring system. In order to overcome such backlash, fairly complex mechanical arrangements are required which increase the cost of the encoder system.
The second approach taken in the prior art to monitor linear travel of machine elements has been the direct mounting of a transparent sheet or other photooptical element with distance gradations or codes marked thereon on a first mechanical element of the machine. A photooptical sensing system is then mounted on a second portion of the overall machine and is used to sense motion of the transparent sheet. Thus, the alignment of the machine elements themselves is utilized to assure alignment between the elements within the linear encoder. This approach is not satisfactory for adding an encoding element to an existing machine or for reducing the cost of the encoding element by manufacturing it as a stand-alone item since the encoder requires a unique mechanical configuration for each pair of machine elements.